A gully is a landform created by running water, eroding sharply into soil, typically on a hillside. Gullies resemble large ditches or small valleys, but are metres to tens of metres in depth and width. When the gully formation is in process, the water flow rate can be substantial, which causes the significant deep cutting action into soil.

Gullying or gully erosion is the process by which gullies are formed. Hillsides are more prone to gullying when they are cleared of vegetation, through deforestation, over-grazing or other means. The eroded soil is easily carried by the flowing water after being dislodged from the ground, normally when rainfall falls during short, intense storms such as during thunderstorms.

Gullies reduce the productivity of farmland where they incise into the land, and produce sediment that may clog downstream waterbodies. Because of this, much effort is invested into the study of gullies within the scope of geomorphology, in the prevention of gully erosion, and in restoration of gullied landscapes. The total soil loss from gully formation and subsequent downstream riversedimentation can be sizeable.

Gullies are widespread at mid- to high latitudes on the surface of Mars, and are some of the youngest features observed on that planet, probably forming within the last few 100,000 years. There, they are one of the best lines of evidence for the recent presence of liquid water on Mars in the recent geological past, probably resulting from the slight melting of snowpacks on the surface[1] or ice in the shallow subsurface[2] on the warmest days of the Martian year. Flow as springs from deeper seated liquid water aquifers in the deeper subsurface is also a possible explanation for the formation of some Martian gullies.[3]

The earliest known usage of the term is from 1657. It originates from the French word goulet, a diminutive form of goule which means throat. It is possible that the term was derived from a type of knife at the time, a gully-knife, because hills that have gullies look as if they are cut open with a sharp knife.[citation needed]